1. Field of the Invention
The present invention relates to a motor circuit with power-off braking function and, more particularly, to a motor circuit that can form a plurality of brake loops to share the transient current during braking.
2. Description of the Related Art
FIG. 1 shows a conventional motor circuit with power-off braking function disclosed in Taiwan Patent Publication No. 201117546 entitled “FAN SYSTEM AND STOP CONTROL CIRCUIT THEREOF”. The motor circuit includes a driving unit 91, a coil unit 92, a braking unit 93, an energy storage unit 94, and a control unit 95. The driving unit 91 includes two electronic switches M1 and M3 connected in series between a high voltage end and a low voltage end, forming a first arm switch unit 911, with a first series contact S1 located between the electronic switches M1 and M3. The driving unit 91 further includes another two electronic switches M2 and M4 connected in series between the high voltage end and the low voltage end, forming a second arm switch unit 912, with a second series contact S2 located between the electronic switches M2 and M4. Thus, the first and second arm switch units 911 and 912 form the driving unit 91 of a full bridge type. The coil unit 92 is comprised of a coil and includes two ends respectively connected to the first and second series contacts S1 and S2. The braking unit 93 includes a loop actuating switch 931 having a high potential port 931a, a low potential port 931b, and a control port 931c. The high potential port 931a is coupled to the high voltage end, the low potential port 931b is coupled to the first series contact S1, and the control port 931c is coupled to a DC power source VCC of the motor circuit. The energy storage unit 94 is a capacitor and includes a power output 941 connected to the high potential port 931a of the loop actuating switch 931. The control unit 95 is directly or indirectly coupled to and, thus, controls the electronic switches M1, M2, M3, and M4.
In a case that the DC power source VCC normally supplies power to the conventional motor circuit, the control unit 95 controls the electronic switches M1 and M4 to be conductive and controls the electronic switches M2 and M3 to be not conductive, such that the current flows from the first series contact S1 through the coil unit 92 to the second series contact S2. Alternatively, the control unit 95 controls the electronic switches M2 and M3 to be conductive and controls the electronic switches M1 and M4 to be not conductive, such that the current flows from the second series contact S2 through the coil unit 92 to the first series contact S1. Thus, an alternating magnetic field is created by the coil unit 92 through provision of the alternating current, driving a rotor of the motor to rotate. Furthermore, since a difference between the potentials of the high potential port 931a and the control port 931c of the loop actuating switch 931 is small, the loop actuating switch 931 is in an open state. On the other hand, if the power from the DC power source VCC is cut of the control unit 95 stops controlling the electronic switches M1, M2, M3, and M4. In this case, the energy storage unit 94 releases electric energy through the power output 941, such that the loop actuating switch 931 and the electronic switch M2 becomes conductive due to electricity supplied from the energy storage unit 94. By such an arrangement, the coil unit 92, the electronic switch M2 and the loop actuating switch 931 form a closed loop that rapidly absorbs the back electromotive force generated at the coil unit 92 resulting from rotation of the rotor, rapidly stopping the operation of the rotor.
However, since the electronic switch M2 of the second arm switch unit 912 must be switched in a high frequency during normal power supply so as to create the alternating current and since the electronic switch M2 must be in a conductive state during the power-off period so as to form the closed loop, the damage rate of the electronic switch M2 is significantly higher than the other electronic switches M1, M3, and M4. Furthermore, when the DC power source VCC normally supplies power to operate the conventional motor circuit, if the loop actuating switch 931 mistakenly turns into the conductive state due to an electromagnetic effect in the alternating magnetic field, an extra surge current is apt to be created and passes through the electronic switch M3. Namely, the current flowing from the electronic switch M2 through the coil unit 92 passes through the electronic switch M3, and the current flowing through the loop actuating switch 931 also passes through the electronic switch M3, leading to damage to the electronic switch M3.
Thus, a need exists for a novel motor circuit with power-off braking function to avoid tremendous load to a single electronic switch due to absorption of the back electromotive force by a single closed loop during the power-off period and to avoid damage to the electronic switches resulting from erroneous actuation during normal power supply, prolonging the service life of the motor circuit with power-off braking function.